ABSTRACT Innate or acquired resistance represents a major limitation of targeted cancer therapy. The broader goal of this application is to develop drugs that prevent resistance to targeted therapy in lung cancer patients. In addition to cell-autonomous mechanisms of resistance, factors in the tumor microenvironment have been shown to block the response to therapy. The microenvironment-mediated resistance to targeted therapy is frequently mediated by Hepatocyte Growth Factor (HGF) which activates its receptor, MET, expressed on cancer cells. The rate-limiting step in HGF/MET signaling is the proteolytic activation of pro-HGF by one of the three serine proteases, matriptase, hepsin or HGFA. To mimic the activity of the endogenous HGF Activator Inhibitors, HAI- 1 and HAI-2, we developed ?triplex? inhibitors of hepsin, matriptase and HGFA from two chemical series of ketobenzothiazole (Kbts) and cyclic urea benzamidines (Cubs). We confirmed that these compounds, just like HAIs, block pro-HGF activation and thus refer to them as synthetic HAIs (sHAIs). Our strong preliminary data confirmed that sHAIs inhibit HGF/MET signaling, prevent HGF-mediated scattering, migration and survival of cancer cells. In addition, we demonstrated that sHAIs overcome therapeutic resistance to marketed drugs which target EGFR or MET in vitro. The goal of this application is to identify sHAI with optimal pharmacological properties and to provide evidence that the lead sHAI prevents/overcomes resistance to targeted therapy in animal models of lung cancer. Because murine HGF does not activate human MET expressed on human cancer cells, all in vivo experiments will be performed in human HGF knock-in mice (hHGF KI) on a SCID background. Our specific aims are: Specific aim 1: To characterize lead sHAIs for their pharmacokinetic properties in vitro and in vivo, and dose- dependent toxicity mice. a) Determine metabolic stability, solubility, plasma protein binding, half-life and clearance of lead sHAIs that have the best combination of potency and target selectivity. b) Perform maximum tolerated dose (MTD) studies on selected sHAIs to prioritize two compounds for in vivo efficacy studies. Specific aim 2: To demonstrate that prioritized sHAIs exert anticancer activity in animal models of lung cancer and overcome resistance to targeted therapy. a) Demonstrate that two lead sHAIs inhibit HGF-dependent tumor formation, using H596 lung cancer cells that form tumors only in the presence of HGF, such as in human HGF knockin mice. b) Show that sHAI overcomes HGF-dependent resistance of lung cancer cells to EGFR inhibitors and to MET kinase inhibitors in hHGF knockin mice. Collectively, these studies will provide a rationale to include sHIAs into treatment regimens to prevent or to overcome HGF-dependent resistance to targeted therapy, and to improve the treatment outcome in lung cancer patients, who do not respond to standard therapy.